It is known to monitor the power consumption of individual electrical appliances, and to transmit power or energy measurements to a centralized monitoring station. Such systems can be used in residential or commercial premises, and allow the user to discover which devices are consuming power; how much power is being consumed; and when. It is also possible to deliver the measurements to the electricity supplier, for metering purposes or other analysis.
Most of the existing equipment for addressing the monitoring market uses wireless communication between the measurement devices and the central (“master”) aggregator. However, it has also been proposed to use power-line communications for this purpose. For example, the HomePlug Powerline Alliance has proposed that a communications protocol be standardised for smart grid/smart energy applications. This will use existing powerline communications technology, so that the specification will be a profile of the IEEE P1901 standard. This is a worldwide standard for the provision of broadband (high-speed) communications over power line networks.
The present inventors have recognised that wireless solutions will suffer from limited communications range, interference, and significant node cost. They have also recognised that a power-monitoring system based on existing high-speed power-line data communications technology is unnecessarily complex, because the measurements that will be transmitted are small in size and they can be transmitted relatively infrequently (for example, once per second). Consequently, the inventors have identified a need for a simpler and more economical power-line based system. They have also identified a more general need for simple, economical, and reliable communications between electrical devices or sensors and a central station, for the purposes of control or monitoring.